1. Field of the Invention
The present invention relates to a new process for the production of polyisocyanates containing uretdione and/or isocyanurate groups by the partial dimerization and/or trimerization (generic term: oligomerization) of the isocyanate groups of organic diisocyanates using catalysts containing trivalent phosphorus and termination of the oligomerization reaction with deactivation of the catalyst by oxidation, to the polyisocyanates obtainable by this process and to their use in polyurethane coating compositions.
2. Description of the Prior Art
The oligomerization of organic diisocyanates using catalysts containing trivalent phosphorus, in particular tertiary phosphines or peralkylated phosphorous acid triamides, is known from a number of publications. Thus, according to DE-OS 1,670,667 or DE-OS 1,954,093, mixtures of aromatic and aliphatic diisocyanates are reacted to provide polyisocyanates containing the isocyanurate groups corresponding to the diisocyanates in the presence of tertiary phosphines. In addition, it is known from DE-OS 1,670,720, 1,934,763, 3,900,053 and U.S. Pat. No. 4,614,785 that the same catalysts also accelerate the dimerization of aliphatic diisocyanates with formation of uretdione structures. According to DE-OS 3,080,513, 3,227,779 and 3,437,635, pure dimers of aliphatic diisocyanates can be obtained by catalysis with peralkylated phosphorous acid triamides, optionally in the presence of H-acidic co-catalysts as described in DE-OS 3,437,635.
To obtain products having specifically reproducible properties, the oligomerization reaction has to be terminated exactly at a predetermined point.
In the processes according to DE-OS 3,030,513 and 3,227,779, the catalyst is distilled off together with excess unreacted diisocyanate at the desired degree of oligomerization. The disadvantage of this procedure is that uncontrolled secondary reactions can occur during distillation because of the high temperatures. In addition, the distillate which contains the active catalyst and, accordingly, has only limited stability in storage has to immediately be further processed.
In most of the known processes described above, these difficulties are overcome by terminating the oligomerization reaction at a certain degree of oligomerization by deactivation of the catalyst before the excess unreacted diisocyanate is separated from the resin by distillation. The catalyst is generally deactivated by the addition of a catalyst poison. The catalyst poisons proposed in the relevant prior publications are generally compounds which react with the catalysts with salt formation. These compounds include alkylating agents such as methyl iodide, dimethyl sulfate and toluene sulfonic acid methyl ester (DE-OS 1,670,667 and 1,670,720); acylating agents such as benzoyl chloride, acetyl chloride, acetanhydride, succinic anhydride and chloroformic acid ester (DE-OS 1,670,667, 1,670,720 and 1,934,763); and acids such as chloroacetic acid, trichloroacetic acid, trifluoroacetic acid, methane sulfonic acid, perfluorobutane sulfonic acid, phosphoric acid, acidic phosphoric acid esters, gaseous hydrogen chloride and compounds giving off hydrogen chloride, e.g., carbamic acid chlorides (DE-OS 1,670,667 and 3,437,635). In addition, phosphine catalysts can be deactivated with, for example, sulfonyl isocyanates (U.S. Pat. No. 4,614,785) and elemental sulfur (DE-OS 1,954,093).
However, when these catalyst poisons are used, numerous difficulties are encountered in practice. In the processes mentioned above, it is disclosed to deactivate the catalyst with an equimolar quantity of a stopper. However, since a certain quantity of catalyst is always consumed during the modification reaction, it is generally difficult to determine the exact quantity of catalyst poison required, i.e., the equimolar quantity. However, if too little of the stopper is used, part of the catalyst remains active which can lead during and after working up by distillation to the same problems which are encountered in the above-described processes which do not use catalyst poisons.
If these difficulties are overcome by using an excess quantity of stopper, the excess can enter the distillate during the removal of unreacted diisocyanate resulting in a distillate which is difficult or impossible to activate for recycling.
The reaction product formed from the catalyst and catalyst poison should have a higher boiling point than the monomeric diisocyanates used, should not decompose under the conditions used for working up by distillation and should therefore remain in the resin. This is generally the case when salt-forming catalyst poisons are used. However, the phosphine sulfides formed where phosphine catalysts are deactivated with elemental sulfur (DOS 1,954,093) are readily volatile under the conditions required for distillation, so that they accumulate as impurities in the distillate, particularly after several recycles.
In addition, some of the described catalyst poisons, for example, the sulfonic isocyanates according to U.S. Pat. No. 4,614,785, have an adverse effect on the color quality of the polyisocyanates formed.
Accordingly, it is an object of the present invention to provide a new process for the oligomerization of diisocyanates using known dimerization and/or trimerization catalysts which are not attended by any of the previously discussed difficulties of the prior art. It is an additional object of the present invention to deactivate the catalyst in such a way that, after the removal of excess monomeric diisocyanates by distillation complete stability of the polyisocyanate and distillate in storage would be obtained. It is a further object of the present invention to provide a resin which retains its reactivity to NCO-reactive groups and a distillate which is free from impurities and, thus, suitable for recycling. It is a final object of the present invention to provide to deactivate the catalyst without causing discoloration or cloudiness of the end product.
Surprisingly, this objects may be achieved in accordance with the process of the present invention which is described in detail hereinafter. The process according to the invention is based on the principle of converting the catalysts containing trivalent phosphorus, in particular tertiary phosphines, into pentavalent, catalytically inactive derivatives, in particular into the corresponding phosphine oxides, by treatment with suitable oxidizing agents.